They seem to count ALL forms of travel (walking, biking, driving). Also the definition of a trip is to go from one address to another. So when I walk over to my neighbors house across the street it is technically a trip.

https://nhts.ornl.gov/vehicle-trips

For vehicle trips:

21.4% of trips are 1 mile or less.

35.2% of trips are 2 miles or less.

45.6% of trips are 3 miles or less.